This invention relates to a liquid level control unit that has particular application in regulating the level of water in a feedwater receiver of a boiler system. The liquid level control unit is adapted to translate changes in the level of a liquid of several inches into small displacements of 3 or 4 thousandths inch. In the preferred embodiment, the translated displacement operates an air valve that controls an air-operated, water control valve for adding make-up water to the feedwater receiver.
The liquid level control unit of this invention is suitable for isolated liquid systems. In a feedwater receiver for a boiler system, the water in the receiver is circulated to a boiler in a circuit that avoids contact with air. In order to translate movement of a internal water displacer device within the receiver tank to an external air valve actuator, the control unit utilizes a magnetic motion transfer mechanism. With this mechanism the internal displacement apparatus is isolated from the external actuator apparatus. The unit does not utilize seals or diaphragms that wear and may eventually leak. Also, the unit is entirely mechanical in operation. In this manner the unit is suitable for use in systems such as a boiler systems where auxiliary components must have a high degree of reliability. It is desirable that the control system be operable during a temporary electrical power failure. Therefore, the unit utilizes a mechanical permanent magnet actuation mechanism to actuate changes in a pressurized air circuit which regulate the water control valve of the boiler system supply water.